Method for producing metallic iron

ABSTRACT

Disclosed is a technique for preventing the adhesive of metallic iron and/or wustite (which is a material produced by the heat reduction of iron oxide contained in a powder derived from an agglomerate that comprises, as a raw material, a mixture containing a iron-oxide-containing substance and a carbonaceous reducing material) on a heath of a movable furnace heath type heating furnace without largely changing the design of a facility for the production, in the production of metallic iron by placing the agglomerate on the heath and heating the agglomerate in the heating furnace to reduce iron oxide contained in the agglomerate. A heath-forming material for preventing the cohesive of metallic iron and/or wustite (which is a material produced by the heat reduction of iron oxide contained in the powder derived from the agglomerate) on the heath is charged into the furnace together with the agglomerate.

TECHNICAL FIELD

The present invention relates to a method for producing massive metalliciron by placing an agglomerate, which is produced from a raw materialmixture containing an iron oxide source, such as iron ore or iron oxide,and a carbon-containing reducing material, on a hearth of a movablehearth furnace and heating the agglomerate to reduce iron oxide in theagglomerate.

BACKGROUND ART

A direct reduction ironmaking process has been developed in whichmetallic iron is produced from a raw material mixture containing an ironoxide source (hereinafter also referred to as an iron-oxide-containingsubstance), such as iron ore or iron oxide, and a carbon-containingreducing material (hereinafter also referred to as a carbonaceousreducing material). In accordance with this ironmaking process, anagglomerate formed of the raw material mixture is placed on a hearth ofa movable hearth furnace and is heated in the furnace utilizing gas heattransfer or radiant heat of a heating burner to reduce iron oxide in theagglomerate with the carbonaceous reducing material, yielding massivemetallic iron. In the ironmaking process, however, part of theagglomerate is pulverized to a powder by rolling, collision, or dropimpact. When the agglomerate is placed on the hearth, the powder derivedfrom the agglomerate accompanies the agglomerate and accumulates on thehearth to form an accumulation layer. Like the agglomerate, theaccumulation layer is heat reduced in the furnace to form metallic ironor wustite (FeO). Metallic iron or wustite left in the furnaceaccumulates on the hearth and raises the hearth level, which makesoperation difficult. To avoid this, the accumulation layer is usuallyremoved with a discharger. However, because of its small thickness, theaccumulation layer on the hearth sometimes remains on the hearth evenafter the removal of massive metallic iron, which was formed by the heatreduction of iron oxide in the agglomerate, from the furnace. Thus, theaccumulation layer is compressed with the discharger and finally forms alarge solid that cannot be discharged from the furnace. Furthermore, thedischarge of a lump formed by the aggregation of metallic iron orwustite from the furnace sometimes forms unevenness on the hearth, whichmakes operation difficult. Patent Literatures 1 to 3 propose a techniquefor solving these problems.

Patent Literature 1 proposes a method for preventing the formation of asteel sheet on a hearth. The method involves the use of a discharger fordischarging reduced iron, which is produced by the reduction of a carboncomposite iron oxide agglomerate, from a movable hearth reductionfurnace and the operation of maintaining the gap between the surface ofthe moving bed and the discharger. According to this technique, the gapcan prevent a powder derived from an agglomerate and accompanying theagglomerate in the furnace from being pressed on the hearth and preventthe formation of a rigid steel sheet.

Patent Literature 2 proposes a method for removing a substance adheredon a hearth of a rotary hearth reduction furnace from the hearthsurface, which involves quenching the hearth surface to cause cracks inthe adhesive material on the hearth before removing the adhesivematerial from the hearth.

Patent Literature 3 proposes a method for maintaining the cleanliness ofthe hearth surface of a rotary hearth furnace by removing a metalliciron powder left on the hearth or adhesives on hearth bricks or bypreventing the metallic iron powder from remaining on the hearth.According to this maintenance method, reduced iron powder left on thehearth is removed from the hearth by blowing off the reduced iron powderwith a gas jet between the outlet for the reduced iron and the inlet forthe raw materials.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 3075721

PTL 2: Japanese Unexamined Patent Application Publication No. 2002-12906

PTL 3: Japanese Unexamined Patent Application Publication No. 11-50120

SUMMARY OF INVENTION Technical Problem

The techniques disclosed in Patent Literatures 1 to 3 require a designchange of the discharger for discharging reduced iron from the movablehearth reduction furnace, the construction of an apparatus for quenchingthe hearth surface, or the construction of an apparatus for blowing offreduced iron powder, which increases the capital investment.

In view of the situations described above, it is an object of thepresent invention to provide a method for producing metallic iron byplacing an agglomerate, which is produced from a raw material mixturecontaining an iron-oxide-containing substance and a carbonaceousreducing material, on a hearth of a movable hearth furnace and heatingthe agglomerate to reduce iron oxide in the agglomerate. This techniqueprevents metallic iron or wustite, which is produced by the heatreduction of iron oxide contained in a powder derived from theagglomerate, from sticking to the hearth without significantly changingthe design for facilities.

Solution to Problem

A method for producing metallic iron according to the present inventionthat can solve the problems described above has a main point in that, inthe production of metallic iron by placing an agglomerate (having aparticle size, for example, in the range of 5 to 50 mm), which isproduced from a raw material mixture containing an iron-oxide-containingsubstance and a carbonaceous reducing material, on a hearth of a movablehearth furnace and heating (for example, 1200° C. to 1400° C.) theagglomerate to reduce iron oxide in the agglomerate, a hearth-formingmaterial for preventing metallic iron and/or wustite, which is producedby heat reduction of iron oxide contained in a powder derived from theagglomerate, from sticking to the hearth is charged into the furnacetogether with the agglomerate.

(a) When the carbon content of the agglomerate is 122% (which hereinmeans % by mass) or more of the carbon content required to reduce ironoxide in the agglomerate, the composition of the hearth-forming materialis preferably controlled such that the CaO, SiO₂, and Al₂O₃ contents ofthe total composition of the powder derived from the agglomerate and thehearth-forming material satisfy the following formulae (1) and (2):

[CaO]/[SiO₂]=0.25 to 1.20   (1)

[Al₂O₃]/[SiO₂]=0.2 to 0.7   (2)

wherein each [ ] in the formulae (1) and (2) denotes the content (% bymass) for the component specified in the square brackets.

In the case of (a), the composition of the hearth-forming material ispreferably controlled such that the total CaO, SiO₂, and Al₂O₃ contentis in the range of 3.0% to 7.0% of the total composition of the powderderived from the agglomerate and the hearth-forming material.

(b-1) When the carbon content of the agglomerate is less than 122% ofthe carbon content required to reduce iron oxide in the agglomerate, thecomposition of the hearth-forming material is preferably controlled suchthat the total carbon content of the total composition of the powderderived from the agglomerate and the hearth-forming material is 122% ormore of the carbon content required to reduce iron oxide in theagglomerate and that the CaO, SiO₂, and Al₂O₃ contents of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material satisfy the following formulae (3) and (4):

[CaO]/[SiO₂]=0.25 to 1.20   (3)

[Al₂O₃]/[SiO₂]=0.2 to 0.7   (4)

wherein each [ ] in the formulae (3) and (4) denotes the content (% bymass) for the component specified in the square brackets.

(b-2) When the carbon content of the agglomerate is less than 122% ofthe carbon content required to reduce iron oxide in the agglomerate, thecomposition of the hearth-forming material is preferably controlled suchthat the total carbon content of the total composition of the powderderived from the agglomerate and the hearth-forming material remainsless than 122% of the carbon content required to reduce iron oxide inthe agglomerate and that the CaO, SiO₂, Al₂O₃, and MgO contents of thetotal composition of the powder derived from the agglomerate and thehearth-forming material satisfy at least one of the following formulae(5) to (9):

[CaO]/[SiO₂]<0.25   (5)

[CaO]/[SiO₂]>1.20   (6)

[Al₂O₃]/[SiO₂]<0.2   (7)

[Al₂O₃]/[SiO₂]>0.7   (8)

[MgO]/[SiO₂]>0.4   (9)

wherein each [ ] in the formulae (5) to (9) denotes the content (% bymass) for the component specified in the square brackets.

In the case of (b-2), the composition of the hearth-forming material ispreferably controlled such that the total CaO, SiO₂, and Al₂O₃ contentis more than 7.0% of the total composition of the powder derived fromthe agglomerate and the hearth-forming material.

A hearth-forming material having a particle diameter in the range of 0.5to 2 mm preferably constitutes 50% by mass or more of the total amountof hearth-forming material charged in the furnace.

Advantageous Effects of Invention

According to the present invention, a hearth-forming material chargedinto a hearth of a movable hearth furnace together with an agglomeratecan prevent metallic iron or wustite, which is produced by heatreduction of iron oxide contained in a powder derived from theagglomerate, from sticking to the hearth. This can prevent a largeadhesive material, such as a steel sheet, that cannot be discharged fromthe furnace from being formed on the hearth and prevent the rising ofthe hearth level. Thus, metallic iron can be efficiently producedwithout significantly changing the design for facilities.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between temperature anddeformation ratio when a pellet is reduced at different [MgO]/[SiO₂]ratios.

FIG. 2 is a photograph substituted for drawing of a cross section of areduced pellet.

FIG. 3 is a graph of 40% contraction temperature as a function of[MgO]/[SiO₂].

FIG. 4 is a graph of 40% contraction temperature as a function of[CaO]/[SiO₂].

FIG. 5 is a SiO₂—MgO—FeO ternary equilibrium diagram.

FIG. 6 is a CaO—SiO₂—MgO ternary equilibrium diagram.

FIG. 7 is a CaO—SiO₂—Al₂O₃ ternary equilibrium diagram.

DESCRIPTION OF EMBODIMENTS

The techniques proposed in Patent Literatures 1 to 3 require significantdesign changes of facilities and significant capital investments. Thus,the present inventors have made extensive studies in order to provide amethod for efficiently producing metallic iron with a minimum capitalinvestment, in which metallic iron or wustite produced by heat reductionof iron oxide contained in a powder derived from an agglomerate in afurnace is prevented from sticking to a hearth, which prevents theformation of a large adhesive material, such as a steel sheet, thatcannot be discharged from the furnace on the hearth and prevents therising of the hearth level. As a result, it was found that ahearth-forming material can be charged into the furnace together withthe agglomerate. More specifically, the present invention was completedby finding that, with consideration given to the carbon content of theagglomerate charged into the furnace and the carbon content required toreduce iron oxide contained in the agglomerate, the hearth-formingmaterial can be charged into the furnace while the composition of thehearth-forming material is appropriately controlled such that the totalcomposition of a powder derived from the agglomerate and thehearth-forming material satisfies predetermined conditions.

A method for producing metallic iron according to the present inventionis characterized in that a hearth-forming material for preventingmetallic iron and/or wustite, which is produced by heat reduction ofiron oxide contained in a powder derived from an agglomerate, fromsticking to a hearth is charged into a furnace together with theagglomerate. The powder derived from the agglomerate adhered on thehearth is based on a powder accompanying the agglomerate charged intothe furnace and a powder produced by disintegration of the agglomeratecaused by rapid heating in the furnace. Thus, when the agglomerate ischarged into the furnace together with a hearth-forming material, thehearth-forming material can be mixed with the powder derived from theagglomerate on the hearth. Appropriate control of the composition of thehearth-forming material in consideration of the composition of thepowder derived from the agglomerate can prevent metallic iron or wustiteproduced by heat reduction of iron oxide contained in the powder derivedfrom the agglomerate from sticking to the hearth. This can prevent theformation of an adhesive material, such as a steel sheet, or the risingof the hearth level, thus increasing the metallic iron productionefficiency.

The hearth-forming material is added before the agglomerate is chargedinto the furnace and preferably when the hearth-forming material isblended with the agglomerate.

When the hearth-forming material is added before the agglomerate ischarged, for example, the hearth-forming material may be added to theagglomerate on a conveyor for charging the agglomerate into a hopper,and a mixture of the agglomerate and the hearth-forming material may beplaced on the hearth. Among the charged mixture, a powder derived fromthe agglomerate and granules of the hearth-forming material accumulateon a lower portion of the agglomerate and move as a mixture when theagglomerate is leveled off with a leveler.

A material for preventing metallic iron or wustite produced by heatreduction of iron oxide contained in a powder derived from anagglomerate from sticking to a hearth may be charged as thehearth-forming material. More specifically, paying attention to thecarbon content of the agglomerate, the hearth-forming material may becharged into the furnace while the composition of the hearth-formingmaterial is controlled in a manner that depends on whether the carboncontent is (a) 122% or more or (b) less than 122% of the carbon contentrequired to reduce iron oxide in the agglomerate. When the carboncontent of the agglomerate is 100% of the carbon content required toreduce iron oxide in the agglomerate, this means that the iron oxide inthe agglomerate is entirely (100%) reduced. When the carbon content is122% of the carbon content required to reduce iron oxide in theagglomerate, this means that the carbon content is in excess of 22%, andthe carbon content of 22% corresponds to approximately 5% of theresidual carbon in the agglomerate after reduction.

The carbon content of the agglomerate and the carbon content required toreduce iron oxide in the agglomerate can be calculated from thecomposition of the raw material mixture composing the agglomerate. Thecarbon content of the agglomerate after heat reduction of iron oxide inthe agglomerate can be determined, for example, by charging theagglomerate into an electric furnace, heating the agglomerate in aninert atmosphere (for example, N₂ atmosphere) at 1300° C.(representative temperature), and measuring the amount of residualcarbon in the agglomerate after reduction reaction by infrared analysis.The carbon content of the agglomerate before heating can be calculatedbackwards from the total of this analytical value and the carbon contentrequired to reduce iron oxide in the agglomerate.

(a) 122% or More

When the carbon content of the agglomerate is 122% or more of the carboncontent required to reduce iron oxide in the agglomerate, thecomposition of the hearth-forming material may be controlled such thatthe CaO, SiO₂, and Al₂O₃ contents of the total composition of the powderderived from the agglomerate and the hearth-forming material satisfy thefollowing formulae (1) and (2):

[CaO]/[SiO₂]=0.25 to 1.20   (1)

[Al₂O₃]/[SiO₂]=0.2 to 0.7   (2)

wherein each [ ] in the formulae (1) to (2) denotes the content (% bymass) for the component specified in the square brackets.

Thus, when the carbon content of the agglomerate is higher than therequired carbon content, and carbon remains after heat reduction, ironoxide contained in the agglomerate is completely reduced, and theresulting metallic iron forms separate fine granules. Furthermore,excessive carbon in the agglomerate promotes the carburization ofmetallic iron through heat reduction, thus separating one metallic irongranule from another with a hard and brittle slag phase. Thus, even ifan adhesive material, such as a steel sheet, is formed on the hearth,the adhesive material can be easily crushed and removed from thefurnace.

Thus, when the carbon content of the agglomerate is 122% or more of therequired carbon content, it is effective to further promote thegranulation of metallic iron so as to facilitate the discharge ofmetallic iron from the furnace. In order to promote the granulation ofmetallic iron, the present invention focuses on slag associated with theproduction of metallic iron, and the melting point of the slag isdecreased to promote the aggregation and granulation of metallic iron.More specifically, the composition of the hearth-forming material iscontrolled such that the CaO, SiO₂, and Al₂O₃ contents of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material satisfy the formulae (1) and (2).

Regarding Formula (1)

When [CaO]/[SiO₂] is preferably in the range of 0.25 to 1.20, themelting point of the slag can be decreased to promote the granulation ofmetallic iron. [CaO]/[SiO₂] is more preferably 0.3 or more and 1.1 orless.

Regarding Formula (2)

When [Al₂O₃]/[SiO₂] is preferably in the range of 0.2 to 0.7, themelting point of the slag can be decreased to promote the granulation ofmetallic iron. [Al₂O₃]/[SiO₂] is more preferably 0.6 or less, still morepreferably 0.4 or less.

When the carbon content of the agglomerate is 122% or more of therequired carbon content, the composition of the hearth-forming materialis preferably controlled such that the total CaO, SiO₂, and Al₂O₃content is in the range of 3.0% to 7.0% of the total composition of thepowder derived from the agglomerate and the hearth-forming material. Ahigher amount of molten slag results in promotion of the carburizationof metallic iron after heat reduction. Thus, when the total amount ofthe components described above is preferably 3.0% or more, thegranulation of metallic iron can be promoted. The total amount is morepreferably 4.5% or more, still more preferably 5.0% or more. However,the total amount of more than 7.0% results in excessively increasedmolten slag, which may flow downward and erode the hearth. Thus, thetotal amount is preferably 7.0% or less, more preferably 6.5% or less.

(b) Less Than 122%

When the carbon content of the agglomerate is less than 122% of therequired carbon content,

(b-1) the composition of the hearth-forming material is controlled suchthat the total carbon content of the total composition of the powderderived from the agglomerate and the hearth-forming material is 122% ormore of the required carbon content, or

(b-2) the composition of the hearth-forming material is controlled suchthat the total carbon content of the total composition of the powderderived from the agglomerate and the hearth-forming material remainsless than 122% of the required carbon content.

In the case of (b-1), it is important that the composition of thehearth-forming material is controlled such that the total carbon contentof the total composition of the powder derived from the agglomerate andthe hearth-forming material is 122% or more of the required carboncontent, and the composition of the hearth-forming material iscontrolled such that the CaO, SiO₂, and Al₂O₃ contents of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material satisfy the following formulae (3) and (4):

[CaO]/[SiO₂]=0.25 to 1.20   (3)

[Al₂O₃]/[SiO₂]=0.2 to 0.7   (4)

wherein each [ ] in the formulae (3) and (4) denotes the content (% bymass) for the component specified in the square brackets.

More specifically, when the carbon content of the agglomerate is lessthan 122% of the required carbon content, this results in slightlyinsufficient carbon, and part of iron oxide contained in the powderderived from the agglomerate may remain unreduced, for example, aswustite. This also results in less carbon involved in the carburizationof metallic iron and promotes the formation of a metallic iron sheetinstead of the granulation of metallic iron. Thus, in order tocompletely reduce iron oxide in the powder derived from the agglomerateand sufficiently carburize the iron oxide for granulation, acarbonaceous reducing material is blended as the hearth-formingmaterial, the deficiency in the carbon content of the agglomerate iscompensated for, and the composition of the hearth-forming material iscontrolled such that the total carbon content of the total compositionof the powder derived from the agglomerate and the hearth-formingmaterial is 122% or more of the required carbon content.

In this case, the CaO, SiO₂, and Al₂O₃ contents of the total compositionof the powder derived from the agglomerate and the hearth-formingmaterial must satisfy the formulae (3) and (4). The formulae (3) and (4)are the same as the formulae (1) and (2) and are defined on the basis ofthe same finding. More specifically, the melting point of the slag canbe decreased to further promote the granulation of the metallic iron,thus facilitating the removal of the metallic iron from the furnace.

In the case of (b-2), it is important that the composition of thehearth-forming material is controlled such that the total carbon contentof the total composition of the powder derived from the agglomerate andthe hearth-forming material remains less than 122% of the requiredcarbon content, and that the CaO, SiO₂, Al₂O₃, and MgO contents of thetotal composition of the powder derived from the agglomerate and thehearth-forming material satisfy at least one of the following formulae(5) to (9):

[CaO]/[SiO₂]<0.25   (5)

[CaO]/[SiO₂]>1.20   (6)

[Al₂O₃]/[SiO₂]<0.2   (7)

[Al₂O₃]/[SiO₂]>0.7   (8)

[MgO]/[SiO₂]>0.4   (9)

wherein each [ ] in the formulae (5) to (9) denotes the content (% bymass) for the component specified in the square brackets.

When no carbonaceous reducing material is blended as the hearth-formingmaterial, and the total carbon content of the total composition of thepowder derived from the agglomerate and the hearth-forming materialremains less than 122% of the required carbon content, it is effectiveto appropriately control the composition of gangue components. Morespecifically, the melting point of the gangue components can beincreased to form solid gangue between metallic iron or wustiteparticles and thereby increase the distance between the metallic iron orwustite particles, which can prevent the aggregation of the metalliciron or wustite particles. This can prevent the metallic iron or wustiteparticles from sticking to the hearth or sticking to the hearth andforming a lump to raise the hearth level.

Metallic iron produced by the reduction of iron oxide contained in thepowder derived from the agglomerate is minute and has very low cohesion.Depending on the composition of gangue components, such as CaO, SiO₂,and Al₂O₃, the resulting slag may have a low melting point, and theformation of molten slag during heat reduction facilitates the movementof Fe atoms on the metallic iron surface in the vicinity of the moltenslag, which promotes the coalescence of metallic iron to form areticulated metallic iron coalescence layer. The compression of themetallic iron coalescence layer results in the formation of a densemetal steel sheet (adhesive material), making it difficult to removemetallic iron from the furnace.

Insufficient reduction of iron oxide also results in the formation ofwustite (FeO). Even in this case, the presence of the molten slagfacilitates the movement of Fe atoms on the wustite surface and promotesthe coalescence of wustite to form coarse wustite particles. The coarsewustite particles forms large blocks with the molten slag and becomedifficult to remove from the furnace.

Thus, it is believed that the prevention of coalescence between metalliciron or wustite particles or between metallic iron particles and wustiteparticles facilitates the removal of metallic iron or wustite from thehearth. On the basis of such findings, when no carbonaceous reducingmaterial is blended as the hearth-forming material, and the total carboncontent of the total composition of the powder derived from theagglomerate and the hearth-forming material remains less than 122% ofthe required carbon content, it is important to increase the meltingpoint of the resulting slag to reduce the formation of molten slag.

Formulae (5) and (6)

When [CaO]/[SiO₂] is preferably less than 0.25 or more than 1.20, theresulting slag can have a high melting point, which can prevent thecoarsening of metallic iron or wustite particles. [CaO]/[SiO₂] is morepreferably 0.20 or less or 1.25 or more.

Formulae (7) and (8)

When [Al₂O₃]/[SiO₂] is preferably less than 0.2 or more than 0.7, theresulting slag can have a high melting point, which can prevent thecoarsening of metallic iron or wustite particles. [Al₂O₃]/[SiO₂] is morepreferably 0.18 or less, still more preferably 0.16 or less, or morepreferably 0.8 or more.

Formula (9)

MgO can reduce the formation of molten slag and prevent the coarseningof metallic iron or wustite particles. Among gangue components, acomponent having a lower melting point melts earlier during temperaturerise. Dissolution of a component that can increase the melting point inthe gangue components solidifies the molten component. Repetition ofthese yields molten gangue. Thus, even when the average composition ofgangue has a high melting point, a bonded substance may be partlyformed. Since MgO can easily diffuse into solid FeO, a higher MgOcontent results in a higher melting point of slag. Thus, MgO can reducethe formation of molten slag.

As is clear from FIG. 5 described below, the melting point of moltenslag changes greatly with [MgO]/[SiO₂]. Thus, the MgO content may becontrolled while the balance between the MgO content and the SiO₂content is taken into consideration. More specifically, when[MgO]/[SiO₂] is preferably more than 0.4, the formation of molten slagcan be reduced to increase solid slag. [MgO]/[SiO₂] is more preferably0.45 or more, still more preferably 0.5 or more. [MgO]/[SiO₂] is 0.9 orless, for example.

At least one of the formulae (5) to (9) may be satisfied to increase themelting point of the resulting slag.

When the carbon content of the agglomerate is less than 122% of therequired carbon content, and the total carbon content of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material remains less than 122% of the required carboncontent, the composition of the hearth-forming material is preferablycontrolled such that the total CaO, SiO₂, and Al₂O₃ content is more than7.0% of the total composition of the powder derived from the agglomerateand the hearth-forming material. When the total amount is more than7.0%, the amount of gangue can be increased, and the solid slag can beincreased. This can prevent metallic iron or wustite from becomingcoarse through aggregation and adhering to the hearth to raise thehearth level. The total amount is more preferably 7.5% or more, stillmore preferably 8% or more. The total amount is 10% or less, forexample.

A material serving as a CaO source, a SiO₂ source, an Al₂O₃ source, or aMgO source may be blended as the hearth-forming material. Examples ofthe CaO source include calcium oxide (CaO) and limestone (maincomponent: CaCO₃). Examples of the SiO₂ source include silica sand andmixtures with other components, such as serpentinite. Examples of theAl₂O₃ source include bauxite and mixtures with other components, such asalumina-containing iron ore. Examples of the MgO source includeMgO-containing slag, Mg-containing substances extracted from seawater,magnesium carbonate (MgCO₃), and dolomite.

In order to control the composition of the hearth-forming material suchthat the total composition of the powder derived from the agglomerateand the hearth-forming material satisfies the requirements describedabove, the mass of the powder derived from the agglomerate must bemeasured.

Examples of the powder derived from the agglomerate include powders oftwo types: a powder that is produced by disintegration of part of anagglomerate produced from a raw material mixture containing aniron-oxide-containing substance and a carbonaceous reducing material ordisintegration due to impact or abrasion of the agglomerate (hereinafteralso referred to as a powder I) or a powder that is produced bydisintegration of the agglomerate during heat reduction in a furnace(hereinafter also referred to as a powder II).

The mass of the powder I is measured, for example, by measuring thetotal mass of agglomerate charged into a furnace and, afterclassification into the agglomerate and the powder derived from theagglomerate, directly measuring the mass of the powder derived from theagglomerate. In the present invention, a powder is defined to have aparticle diameter of 3 mm or less.

The method for directly measuring the mass of the powder derived fromthe agglomerate cannot be applied to cases where the characteristics ofthe agglomerate vary while the agglomerate is continuously charged intoa furnace. As described in an example described below, a rotationstrength test that simulates a transfer process up to charging a formedagglomerate into a furnace may be performed to measure the mass of apowder having a particle diameter of 3 mm or less and estimate the massof the powder derived from the agglomerate.

The mass of the powder II may be measured by heating the agglomerate inan electric furnace and measuring the mass of a powder having a particlediameter of 3 mm or less produced by rapid heating (for example, aheating rate of 10° C./min or more) to estimate the mass of the powderderived from the agglomerate.

On the basis of such estimation of the mass of the powder derived fromthe agglomerate, the total composition of the powder derived from theagglomerate and the hearth-forming material can be represented by thefollowing formulae (21) to (24):

CaO(kg/h):H_(CaO)=(L_(CaO)×W_(L)+C_(CaO)×CW_(L)+S_(CaO)×SW_(L)+A_(CaO)×AW_(L)+M_(CaO)×MW_(L))/100  (21)

SiO₂(kg/h):H_(SiO2)=(L_(SiO2)×W_(L)+C_(SiO2)×CW_(L)+S_(SiO2)×SW_(L)+A_(SiO2)×AW_(L)+M_(SiO2)×MW_(L))/100  (22)

Al₂O₃(kg/h):H_(Al2O3)=(L_(Al2O3)×W_(L)+C_(Al2O3)×CW_(L)+S_(Al2O3)×SW_(L)+A_(Al2O3)×AW_(L)+M_(Al2O3)×MW_(L))/100  (23)

MgO(kg/h):H_(MgO)=(L_(MgO)×W_(L)+C_(MgO)×CW_(L)+S_(MgO)×SW_(L)+A_(MgO)×AW_(L)+M_(MgO)×MW_(L))/100  (24)

In the formulae (21) to (24), L_(CaO), L_(SiO2), L_(Al2O3), and L_(MgO)denote the percentage (% by mass) of CaO, SiO₂, Al₂O₃, and MgO,respectively, in the agglomerate, and W_(L) denotes the mass (kg) of thepowder derived from the agglomerate charged into a furnace per unit time(h).

C_(CaO), C_(SiO2), C_(Al2O3), and C_(MgO) denote the percentage (% bymass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the CaO source ofthe hearth-forming material, and CW_(L) denotes the mass (kg) of the CaOsource in the hearth-forming material charged into a furnace per unittime (h).

S_(CaO), S_(SiO2), S_(Al2O3), and S_(MgO) denote the percentage (% bymass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the SiO₂ source ofthe hearth-forming material, and SW_(L) denotes the mass (kg) of theSiO₂ source in the hearth-forming material charged into a furnace perunit time (h).

A_(CaO), A_(SiO2), A_(Al2O3), and A_(MgO) denote the percentage (% bymass) of SiO₂, CaO, Al₂O₃, and MgO, respectively, in the Al₂O₃ source ofthe hearth-forming material, and AW_(L) denotes the mass (kg) of theAl₂O₃ source in the hearth-forming material charged into a furnace perunit time (h).

M_(CaO), M_(SiO2), M_(Al2O3), and M_(MgO) denote the percentage (% bymass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the MgO source ofthe hearth-forming material, and MW_(L) denotes the mass (kg) of the MgOsource in the hearth-forming material charged into a furnace per unittime (h).

When the target compositions are represented by the following formulae(25) to (28), the total composition of the powder derived from theagglomerate and the hearth-forming material is represented by thefollowing formulae (29) to (32) on the basis of the formulae (21) to(24):

[CaO]/[SiO₂]=1.3   (25)

[Al₂O₃]/[SiO₂]=0.3   (26)

[MgO]/[SiO₂]=0.5   (27)

CaO+Al₂O₃+SiO₂=7   (28)

H_(CaO)/H_(SiO2)=1.3   (29)

H_(Al2O3)/H_(SiO2)=0.3   (30)

H_(MgO)/H_(SiO2)=0.5   (31)

(H_(CaO)+H_(Al2O3)+H_(SiO2))/(W_(L)+CW_(L)+SW_(L)+AW_(L)+MW_(L))×100=7  (32)

Since no SiO₂ source is generally added as the hearth-forming material,SW_(L) may be 0. When the SiO₂ source is added, the SiO₂ source is setat a temporary value, and another additive amount that gives the targetcomponent ratio is determined. When the result does not reach the targetvalue, the amount of SiO₂ source to be added is changed until thesolution is obtained.

With respect to the hearth-forming material, a hearth-forming materialhaving a particle diameter in the range of 0.5 to 2 mm preferablyconstitutes 50% by mass or more of the total amount of hearth-formingmaterial charged into a furnace. Although the hearth-forming materialhaving a smaller particle diameter is easier to mix with the powderderived from the agglomerate, the hearth-forming material having anexcessively small particle diameter is blown off by wind while thehearth-forming material is charged into a furnace or heated in thefurnace and cannot achieve the intended effects. Thus, thehearth-forming material having a particle diameter of 0.5 mm or morepreferably constitutes 50% by mass or more. However, the hearth-formingmaterial having an excessively large particle diameter is difficult tomix with the powder derived from the agglomerate and cannot achieve theintended effects. In order to rapidly melt CaO or MgO in molten ganguewhen the gangue begins to melt and thereby promote the solidification ofslag, it is recommended that the hearth-forming material have anincreased surface area. Thus, the hearth-forming material having aparticle diameter of 2 mm or less preferably constitutes 50% by mass ormore.

The agglomerate is produced by shaping a raw material mixture containingan iron-oxide-containing substance and a carbonaceous reducing material.The iron-oxide-containing substance may be iron ore, iron sand, or anonferrous smelting residue. The carbonaceous reducing material may be acarbon-containing substance, for example, coal or coke.

The raw material mixture may contain another component, such as abinder, a MgO source, or a CaO source. The binder may be apolysaccharide (for example, starch, such as wheat flour). The MgOsource or the CaO source may be one exemplified as the MgO source or theCaO source to be blended into the hearth-forming material.

The agglomerate may have any shape, for example, a pellet or briquetteform. The agglomerate may have any size and may have a particle size(maximum diameter) of 50 mm or less. The particle size is approximately5 mm or more. The particle size of the agglomerate in a briquette formmay be a sphere-equivalent diameter.

The agglomerate may be heated in the furnace to an agglomeratetemperature in the range of 1200° C. to 1400° C. to reduce iron oxide inthe raw material mixture.

The furnace may be a movable hearth furnace, for example, a rotaryhearth furnace.

The temperature of the agglomerate is particularly preferably 1250° C.or more. At an agglomerate temperature of 1250° C. or more, the meltingtime of metallic iron and slag can be decreased. However, an excessivelyhigh agglomerate temperature may result in erosion of a hearth withmolten metallic iron, raising the hearth level. Thus, the temperature ofthe agglomerate is preferably 1350° C. or less.

The agglomerate may be heated with a burner, and the temperature of theagglomerate may be controlled with the combustion conditions of theburner.

Although the present invention is more specifically described in thefollowing examples, the present invention is not limited to theseexamples. Various modifications may be made to these examples withoutdeparting from the gist described above and below. These modificationsare also within the technical scope of the present invention.

EXAMPLES

In Experimental Example 1, an agglomerate produced from a raw materialmixture containing an iron-oxide-containing substance and a carbonaceousreducing material was heated in a furnace to reduce iron oxide in theraw material mixture, producing metallic iron. The composition andstrength of the metallic iron were determined to examine therelationship between fixation on a hearth and the composition. InExperimental Example 2, the effects of CaO, SiO₂, and MgO on thedeformation ratio of agglomerate were examined to determine therelationship between the composition and the formation behavior ofmolten slag. In Experimental Example 3, a ternary phase diagram was usedto examine the relationship between the melting temperature of an Al₂O₃slag component and the composition.

Experimental Example 1

An agglomerate having the composition listed in the following Table 1was produced as an agglomerate produced from a raw material mixturecontaining an iron-oxide-containing substance and a carbonaceousreducing material. The shape of the agglomerate was a pillow-shapedbriquette having a sphere-equivalent diameter (maximum diameter) in therange of approximately 22 to 26 mm for Nos. 1, 6, and 7 and a sphericalpellet having a particle size (maximum diameter) in the range ofapproximately 12 to 20 mm for Nos. 2 to 5 in Table 1. In Table 1, TFedenotes the total iron content, TC denotes the carbon content (in Table1, the total carbon content of the agglomerate), and FC denotes thepercentage of carbon that is not converted into gas at 970° C. Table 1lists [CaO]/[SiO₂], [Al₂O₃]/[SiO₂], [MgO]/[SiO₂], and[CaO]+[Al₂O₃]+[SiO₂] based on the composition of the agglomerate.

The agglomerate was heated in a furnace to 1300° C. to reduce iron oxidecontained in the agglomerate, thus yielding metallic iron. The heatingtime in the furnace was listed in the following Table 2.

Table 2 listed the measurements of the composition of the agglomerateafter heating. In Table 2, MFe denotes the metallic iron content, TCdenotes the carbon content (in Table 2, the total carbon content afterheating), TC/TFe×100 denotes the ratio of the total carbon content tothe total iron content, and Metal Fe denotes the metallization ratio[=metallic iron content (%)/total iron content (%)×100].

In Table 2, RCs denotes the carbon content of the residue (agglomerate)after heating based on the carbon content of the agglomerate beforeheating. Subtracting RCs from TC of the agglomerate before heatingyields the carbon content used for reduction (RedC). Table 2 lists theratio of the residual carbon after heating to carbon required forreduction (RCs/RedC×100). As is clear from Table 2, the carbon contentat which the residual carbon after heating is approximately 5% of thecarbon content required for reduction is approximately 22%.

The strength of massive metallic iron (agglomerate) after heating wasmeasured in a rotation strength test.

Rotation Strength Test

The residue was sieved in a rotating container at a total number ofrevolutions of 500 in accordance with three particle diameters: 1 mm orless, more than 1 mm and 2 mm or less, and more than 2 mm. The rotatingcontainer is a cylinder having a diameter of 113 mm and a length of 205mm and has two barrels, which rotate at a rotation speed of 30 rpm.

Table 2 lists the ratio of a powder having a particle diameter of 1 mmor less to the mass of the sieved powders. An increase in the percentageof the powder having a particle diameter of 1 mm or less indicates thatthe residue can be easily pulverized, is not adhered on a hearth, andhas good removability. In the present invention, removability isconsidered to be excellent when the percentage of the powder having aparticle diameter of 1 mm or less is 29% or more (working examples) andpoor when the percentage is less than 29% (comparative examples).

The following are discussions based on Table 2. With respect to Nos. 1,2, 3, and 5, the carbon content of the residue is 5% or more(RCs/RedC×100 is 22% or more), and the carbon content of the agglomerateis 122% or more of the carbon content required to reduce iron oxidecontained in the agglomerate. With respect to Nos. 1, 2, and 3 of these,among the compositions of the agglomerate, [CaO]/[SiO₂] is in the rangeof 0.25 to 1.20, and [Al₂O₃]/[SiO₂] is in the range of 0.2 to 0.7, whichsatisfy the formulae (1) and (2). Thus, Nos. 1, 2, and 3 are weaklyadhered on the hearth. In contrast, with respect to No. 5, among thecompositions of the agglomerate, [CaO]/[SiO₂] is 0.23, which does notsatisfy the formula (1). No. 5 also has a total CaO, Al₂O₃, and SiO₂content of less than 3.0%, and metallic iron is easily sintered. Thus,residue removability is not improved.

With respect to Nos. 4, 6, and 7, the carbon content of the residue isless than 5% (RCs/RedC×100 is less than 22%), and the carbon content ofthe agglomerate is less than 122% of the carbon content required toreduce iron oxide contained in the agglomerate. With respect to No. 6 ofthese, among the compositions of the agglomerate, [CaO]/[SiO₂] is 0.14,which satisfies the formula (5). Thus, slag has an increased meltingpoint, and the residue has low cohesion, is easily separable, and hasgood removability. With respect to Nos. 4 and 7, among the compositionsof the agglomerate, [CaO]/[SiO₂] is in the range of 0.25 to 1.20,[Al₂O₃]/[SiO₂] is in the range of 0.2 to 0.7, and [MgO]/[SiO₂] is 0.4 orless, which do not satisfy the formulae (5) to (9). Thus, Nos. 4 and 7are strongly sticked on the hearth.

TABLE 1 Composition of agglomerate (% by mass) [CaO] + [Al₂O₃] + No. TFeFeO TC FC CaO SiO₂ Al₂O₃ MgO [CaO]/[SiO₂] [Al₂O₃]/[SiO₂] [MgO]/[SiO₂][SiO₂] 1 46.24 0.32 21.55 17.35 2.11 2.44 1.44 0.31 0.86 0.59 0.13 5.992 47.95 0.34 20.65 17.99 1.98 2.50 1.49 0.25 0.79 0.60 0.10 5.97 3 52.193.42 16.61 14.47 1.14 1.96 1.33 0.08 0.58 0.68 0.04 4.43 4 53.68 25.0217.10 14.96 1.32 2.22 0.92 0.40 0.59 0.41 0.18 4.46 5 53.57 23.09 17.5315.34 0.38 1.62 0.61 0.29 0.23 0.38 0.18 2.61 6 46.96 1.21 16.21 10.950.55 3.82 1.62 0.12 0.14 0.42 0.03 5.99 7 46.96 0.72 16.18 10.93 1.865.54 1.56 0.12 0.34 0.28 0.02 8.96

TABLE 2 Time Composition after heating (% by mass) Percentage of Carboncontent (% by mass) No. (min) TFe FeO MFe TC TC/TFe × 100 MetalFe 1 mmor less (%) RCs Red C RCs/Red C × 100 1 16.00 80.1 0.51 79.47 11.0 13.7399.2 68.30 6.37 15.18 42 2 16.00 81.1 0.28 80.16 10.5 12.95 98.8 86.206.21 14.44 43 3 16.00 87.2 0.53 86.58 5.0 5.73 99.3 29.31 2.98 13.63 224 16.00 88.0 0.50 87.20 3.6 4.09 99.1 7.12 2.19 14.91 15 5 16.00 86.60.40 86.26 5.1 5.89 99.6 0.40 3.16 14.37 22 6 9.95 84.1 0.90 83.11 3.13.69 98.8 67.24 1.73 14.48 12 7 8.83 80.9 0.92 80.13 1.3 1.61 99.0 5.820.75 15.43 5 MetalFe = MFe/TFe × 100

Experimental Example 2

It is difficult to accurately observe the formation behavior of moltenslag in the reduction of iron oxide in the presence of CaO, SiO₂, andMgO. Because of the coexistence of solid and liquid states andnonuniform presence of each oxide, it is not clear what state of moltenslag contributes to metallic iron sintering and the promotion of coarsecoalescence of wustite.

A MgO source magnesite and a CaO source limestone were blended with ironore containing SiO₂ as a gangue component to form a pellet(agglomerate). The pellet was fired in the air in an electric furnace at1300° C. for 10 minutes. While the pellet was reduced with a gas, thedeformation ratio of the pellet was measured. The effects of CaO, SiO₂,and MgO on the deformation of the pellet were examined. The results weredescribed in “High Temperature Reduction and Softening Properties ofPellets with Magnesite” (Transactions of the Iron and Steel Institute ofJapan, issued by The Iron and Steel Institute of Japan, vol. 23 (1983),No. 2, p. 153).

The reduction of the fired pellet with a gas was performed under a loadof 0.5 kg/pellet with a reducing gas (CO gas:N₂ gas=30% by volume:70% byvolume) while the fired pellet was heated to 1500° C. at 10° C./min. TheSiO₂ content of the pellet was 0.3%, and [MgO]/[SiO₂] was changed in therange of 0.01 to 1.32. The deformation ratio of the pellet was measuredbefore and after reduction. The results were described in the literaturedescribed above. FIG. 1 shows the results.

The deformation of the pellet is based on contraction resulting from thereduction of iron oxide to metallic iron and deformation resulting fromthe formation of molten slag. Deformation at 1100° C. or morepredominantly results from the latter deformation. This is demonstratedby observing structure photographs of a cross section of the pelletdescribed in the literature. FIG. 2 shows structure photographsdescribed in the literature. FIG. 2 shows photographs substituted fordrawing of a cross section of a pellet reduced by heating a fired pelletin which SiO₂ is 4.5% and [MgO]/[SiO₂]=0.59 to 1300° C. FIG. 2(1) showsthe outer area of the pellet, and FIG. 2(2) shows the inner area of thepellet. (1) includes much metallic iron shown in white, and (2) includeswustite shown in gray and molten slag shown in black. The wustiteparticles are coarse and have a molten round surface. Thus, it is clearthat deformation at 1100° C. or more results from the formation ofmolten slag.

In the literature, the effects of the pellet composition on thedeformation of the pellet are examined by measuring the temperature for40% contraction (hereinafter also referred to as 40% contractiontemperature) of the pellet. FIGS. 3 and 4 show the results.

FIG. 3 shows the results for different [MgO]/[SiO₂] with a SiO₂ contentof 4.4% (circles) or 8.3% (crosses) without CaO. FIG. 4 shows theresults for different [CaO]/[SiO₂] with [MgO]/[SiO₂]=0.72.

As is clear from FIG. 3, in the absence of CaO, the 40% contractiontemperature monotonously increases with [MgO]/[SiO₂]. In the presence ofCaO, the 40% contraction temperature is lowest at [CaO]/[SiO₂]=0.45. Thecomposition at which the 40% contraction temperature is 1350° C. is[MgO]/[SiO₂] of more than 0.4 in the absence of CaO as illustrated inFIG. 3 or [CaO]/[SiO₂] of less than 0.18 or more than 1.05 in thepresence of CaO as illustrated in FIG. 4. Although this result isdifferent from the range described above in the presence of CaO and MgO,regarding the results of Example 1 as important, [CaO]/[SiO₂] is definedto be less than 0.25 or more than 1.20.

It is also qualitatively clear from a ternary equilibrium diagram thatsuch a way of thinking is reasonable.

FIG. 5 is a SiO₂—MgO—FeO ternary equilibrium diagram. In FIG. 5,[MgO]/[SiO₂]=0.4 gives a straight line, and the melting point under thiscondition is 1450° C. even for different FeO contents, indicating thatthe melting point monotonously decreases with [MgO]/[SiO₂].

Even having a melting point of 1450° C., for example, all the ganguecomponents are not necessarily solid at 1350° C. Since part of thegangue components melt at approximately 1200° C. or more, a highermelting point is only indicative of a smaller amount of melt.

FIG. 6 is a CaO—SiO₂—MgO ternary equilibrium diagram. In FIG. 6,[MgO]/[SiO₂]=0.4, [MgO]/[SiO₂]=0.72, [CaO]/[SiO₂]=0.25,[CaO]/[SiO₂]=0.45, and [CaO]/[SiO₂]=1.20 give straight lines. Even when[MgO]/[SiO₂] is constant, with a change in [CaO]/[SiO₂], [CaO]/[SiO₂] of0.45 almost results in a compound CaO.MgO.2SiO₂ having a melting pointof approximately 1400° C. Thus, getting closer to the composition of alow-melting-point compound, the molten gangue increases. As shown by thedotted lines in FIG. 6, in order for the melting point of the slag to beapproximately 1450° C. or more, [CaO]/[SiO₂] may be less than 0.25 ormore than 1.20.

Experimental Example 3

The effects of Al₂O₃ on the deformation of the pellet was examined inExperimental Example 2.

FIG. 7 is a CaO—SiO₂—Al₂O₃ ternary equilibrium diagram. In FIG. 7,[Al₂O₃]/[SiO₂]=0.2, [Al₂O₃]/[SiO₂]=0.7, [CaO]/[SiO₂]=0.25, and[CaO]/[SiO₂]=1.20 give straight lines. In a region surrounded by thesestraight lines, a low-melting-point slag having a melting point ofapproximately 1250° C. is partly formed. Outside this region is ahigh-melting-point region. Thus, the amount of molten slag can bedecreased in the outside of this region.

INDUSTRIAL APPLICABILITY

According to the present invention, a large adhesive material, such as asteel sheet, that cannot be discharged from a furnace can be preventedfrom being formed on a hearth, and the hearth level can be preventedfrom being raised. Thus, metallic iron can be efficiently producedwithout significantly changing the design for facilities.

1. A method for producing metallic iron, the method comprising: placingon a hearth of a movable hearth furnace an agglomerate comprising a rawmaterial mixture comprising a carbonaceous reducing material and asubstance comprising iron oxide, and heating the agglomerate to reduceiron oxide in the agglomerate, wherein a hearth-forming material thatprevents metallic iron, wustite, or both, from sticking to the hearth ischarged into the furnace together with the agglomerate, and wherein themetallic iron, wustite, or both, are produced by heat reduction of ironoxide in a powder derived from the agglomerate.
 2. The method of claim1, wherein, when a carbon content of the agglomerate is 122 mass % ormore of a carbon content required to reduce the iron oxide in theagglomerate, the hearth-forming material has a composition such thatCaO, SiO₂, and Al₂O₃ contents of a total composition of the powderderived from the agglomerate and the hearth-forming material satisfyformulae (1) and (2):[CaO]/[SiO₂]=0.25 to 1.20   (1)[Al₂O₃]/[SiO₂]=0.2 to 0.7   (2) wherein each [ ] in the formulae (1) and(2) denotes the content (% by mass) in mass % for the componentspecified in the square brackets.
 3. The method of claim 1, wherein whena carbon content of the agglomerate is less than 122 mass % of a carboncontent required to reduce the iron oxide in the agglomerate, thehearth-forming material has a composition such that a total carboncontent of a total composition of the powder derived from theagglomerate and the hearth-forming material is 122% or more of thecarbon content required to reduce the iron oxide in the agglomerate, andCaO, SiO₂, and Al₂O₃ contents of the total composition of the powderderived from the agglomerate and the hearth-forming material satisfyformulae (3) and (4):[CaO]/[SiO₂]=0.25 to 1.20   (3)[Al₂O₃]/[SiO₂]=0.2 to 0.7   (4) wherein each [ ] in the formulae (3) and(4) denotes the content (% by mass) in mass % for the componentspecified in the square brackets.
 4. The method of claim 1, wherein whena carbon content of the agglomerate is less than 122 mass % of a carboncontent required to reduce the iron oxide in the agglomerate, thehearth-forming material has a composition such that a total carboncontent of a total composition of the powder derived from theagglomerate and the hearth-forming material remains less than 122% ofthe carbon content required to reduce the iron oxide in the agglomerate,and CaO, SiO₂, Al₂O₃, and MgO contents of the total composition of thepowder derived from the agglomerate and the hearth-forming materialsatisfy at least one formula selected from the group consisting offormulae (5) to (9):[CaO]/[SiO₂]<0.25   (5)[CaO]/[SiO₂]>1.20   (6)[Al₂O₃]/[SiO₂]<0.2   (7)[Al₂O₃]/[SiO₂]>0.7   (8)[MgO]/[SiO₂]>0.4   (9) wherein each [ ] in the formulae (5) to (9)denotes the content (% by mass) in mass % for the component specified inthe square brackets.
 5. The production method according to of claim 2,wherein the hearth-forming material has a composition such that a totalCaO, SiO₂, and Al₂O₃ content is 3.0% to 7.0% by mass of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material.
 6. The method of claim 4, wherein thehearth-forming material has a composition such that a total CaO, SiO₂,and Al₂O₃ content is more than 7.0% by mass of the total composition ofthe powder derived from the agglomerate and the hearth-forming material.7. The production method according to claim 1, wherein at least 50% bymass of the hearth-forming material has a particle diameter of 0.5 to 2mm.
 8. The method of claim 1, wherein the agglomerate has a particlesize of 5 to 50 mm.
 9. The method of claim 1, wherein the agglomerate isheated in the movable hearth furnace at a temperature of 1200° C. to1400° C.
 10. The method of claim 2, wherein [CaO]/[SiO₂] is 0.3 to 1.1.11. The method of claim 2, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.6.
 12. Themethod of claim 2, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.4.
 13. The methodof claim 3, wherein [CaO]/[SiO₂] is 0.3 to 1.1.
 14. The method of claim3, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.6.
 15. The method of claim 3,wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.4.
 16. The method of claim 5, whereinthe total CaO, SiO₂, and Al₂O₃ content is 4.5% to 6.5% by mass of thetotal composition of the powder derived from the agglomerate and thehearth-forming material.
 17. The method of claim 5, wherein the totalCaO, SiO₂, and Al₂O₃ content is 5.0% to 6.5% by mass of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material.
 18. The method of claim 6, wherein the totalCaO, SiO₂, and Al₂O₃ content is more than 7.5% by mass of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material.
 19. The method of claim 6, wherein the totalCaO, SiO₂, and Al₂O₃ content is more than 8% by mass of the totalcomposition of the powder derived from the agglomerate and thehearth-forming material.
 20. The method of claim 6, wherein the totalCaO, SiO₂, and Al₂O₃ content is more than 7.0% and less than 10% by massof the total composition of the powder derived from the agglomerate andthe hearth-forming material.